The present invention relates to a transmitter and a receiver for a tire condition monitoring apparatus that permits a driver in a vehicle passenger compartment to check the conditions of tires, such as the air pressure.
Wireless tire condition monitoring apparatuses that allow a driver in a vehicle passenger compartment to check the conditions of vehicle tires have been proposed. For example, Japanese Laid-Open Patent Publication No. 2001-174357 discloses such an apparatus. The apparatus of the publication includes transmitters and a receiver. Each transmitter is located in the wheel of one of the tires and the receiver is located in the body frame of the vehicle. Each transmitter detects conditions, such as air pressure and the temperature of the associated tire, and wirelessly transmits the detected information. The receiver receives data wirelessly transmitted by the transmitters with an antenna and displays the conditions of the tires, for example, on a display located in front of the driver's seat.
The transmitter is constructed of electronic components such as a pressure sensor and a transmission antenna, and those electronic components are packaged on a substrate. The substrate on which electronic components have been packaged is housed within a casing. The casing has a valve stem for filling the tire with air. The valve stem is installed in an installation hole of the wheel. At this time, the casing is installed in a state in which it has abutted against a drop center in the wheel. Therefore, the normal direction to a diaphragm in a pressure sensor for measuring air pressure within the tire becomes the same as the direction of centrifugal force generated as the tire rotates (See Japanese Patent Laid-Open No. 2001-174357).
However, when the normal direction to the diaphragm in the pressure sensor is the same direction as the direction of centrifugal force that is generated as the tire rotates, an error may occur in the value of the air pressure measured by the pressure sensor. In other words, when vehicle speed is under a predetermined speed, the measured air pressure value is not affected by the centrifugal force that is generated as the tire rotates, but when the vehicle speed exceeds the predetermined speed, the centrifugal force that is generated as the tire rotates also becomes greater.
When more than a predetermined value of centrifugal force is applied to the diaphragm 32a of the pressure sensor 32, the diaphragm 32a of the pressure sensor 32 deflects inwardly or outwardly, as shown in FIGS. 5(a) and 5(b). Specifically, when the centrifugal force is applied to the pressure sensor 32 packaged on a lower surface of the substrate 80 from the outside of the diaphragm 32a toward the inside thereof as shown in FIG. 5(a), the diaphragm 32a deflects inwardly. As a result, the measured value is higher than the actual air pressure as shown in FIG. 5(c).
On the other hand, when the centrifugal force is applied to the pressure sensor 32 packaged on an upper surface of the substrate 80 from the inside of the diaphragm 32a toward the outside thereof as shown in FIG. 5(b), the diaphragm 32a deflects outwardly. As a result, the measured value is lower than the actual air pressure as shown in FIG. 5(c). Therefore, when the vehicle speed exceeds the predetermined speed, the centrifugal force that is generated as the tire rotates is applied to the diaphragm 32a of the pressure sensor 32, and a slight error occurs in the measured air pressure data.
The present invention has been achieved by focusing attention on such problems, and an object is to provide a transmitter and a receiver for a tire condition monitoring apparatus capable of reducing measurement error due to the centrifugal force that is generated as the tire rotates.